The present invention relates to a drawing device, and more particularly, to a deep drawing press, having a pressure box which is used as an abutment for generating a clamping force for holding a sheet edge and which is disposed to be movable in the drawing direction by a defined drawing depth, having a force generating device which, on one end, is connected with the pressure box and which, on the other end, is supported on a stationary bearing, for acting upon the pressure box by means of a force parallel to the drawing direction, having force transmitting elements for transmitting the clamping force to a sheet holding device.
As a rule, drawing tools have a sheet holding ring which is assigned to the bottom tool and presses the edge of the metal sheet to be drawn before the start of the actual drawing operation against a corresponding abutment of the top tool fastened to the lowering slide. Thereby, the metal sheet ring is held fast. In this arrangement, the sheet holding ring must be pressed in a defined manner against the abutment on the top tool, while the slide continues to be lowered. Optionally, the adjustment of different clamping forces may be desired at different points of the sheet holding ring in order to, for example, influence the flow of material. In addition to the local change of the flow of force, a time variation of the flow of force is frequently also required, for example, a decreasing sheet holding force may be required with an increasing drawing depth.
DE 40 16 838 A1 describes a drawing device which has a hydraulically vertically adjustable pressure box. For the height adjustment, four servohydraulically controlled pressure cylinders are used which are each arranged on the corners of the pressure box which is rectangular in view. The pressure cylinders have a working stroke which essentially corresponds to the required drawing depth. Studs are provided for transmitting the contact pressure force from the pressure box to the blank holding ring. These studs extend through openings of a stationarily disposed table plate which is arranged between the sheet holding ring and the pressure box. Hydraulic short-stroke cylinders are arranged on the pressure box in which one piston respectively is disposed in a vertically displaceable manner. A stud or a stud position is assigned to each piston. The short-stroke cylinders are combined by groups and connected with a control device which permits the servohydraulic control of fast movements. In addition, the hydraulic short-stroke pistons permit an automatic length compensation of possible length differences of the studs.
For hydraulic lowering and lifting of the pressure box in the timing of the press, however, a relatively efficient source is required for filling the corresponding hydraulic cylinders. Control of the total applied holding force as a rule takes place by throttling devices and is therefore a function of the speed. The slide speed changes during the drawing operation so that a constant readjustment is required. In addition, the stiffness of the hydraulic drive requires a permanent and fast control.
DE 40 16 838 A1 describes a drawing apparatus in which the pressure box is not force-controlled but path-controlled. For this purpose, a vertically displaceable pressure box called a pressure cheek is acted upon by a pressure rod with a vertically upwardly directed force. Distance bolts are provided on both sides of the pressure cheek and are aligned with distance bolts provided on the slide. When the slide moves downward, its associated distance bolts are placed on the distance bolts of the pressure cheek and press the latter downward against the force of the pressure cheeks which is not additionally controlled or directed. Thus, a fixed distance is adjusted between the slide and the pressure cheek. The pressure cheek carries a group of hydraulic cylinders which act upon the sheet holding ring by way of pressure slides. The hydraulic cylinders are connected to pressure control devices to make adjustment of the force transmitted by way of the respectively actuated stud to the sheet holding ring possible. As a rule, hydraulic proportional valves are used for this purpose. Because of the occurring shock-related stress, however, the arrangement of the hydraulic proportional valves on the pressure cheek is critical.
It is an object of the present invention to provide a robust drawing device which permits at least the control of the sheet holding force as a function of the time or as a function of the drawing path.
This object is achieved by a drawing device according to the present invention providing a force generating device with a controllable pneumatic driving device.
More specifically, the drawing device according to the invention has a pressure box which is supported on a pneumatic driving device. This driving device can be controlled with respect to the development of its force so that the sheet holding force can be adjusted as a function of the drawing depth. Apart from acceleration (inertia) forces, the sheet holding force therefore depends solely on the pressures acting on the pneumatic driving device. When, for example, different pressure levels are to be adjusted, it is sufficient to connect the pneumatic driving device with pressure reservoirs which have a correspondingly different pressure. The buffer effect or spring effect inherent to the pneumatic driving device prevents a short-time excessive rise of force. In contrast to hydraulic drives, a corresponding control device for controlling the force generated by the force generating device (driving device) is subjected to much lower demands.
Whereas, in hydraulic systems, during the drawing stroke, the sheet holding force is applied by the controlled throttled discharges of hydraulic fluid from corresponding hydraulic cylinders and, during the return stroke of the slide, the pressure box must be moved upwards again hydraulically, for which relatively high-capacity pumps are required, the pneumatic driving device permits this operation to take place by utilizing correspondingly large buffer volumes.
In a particularly advantageous embodiment of the present invention, the pneumatic driving device is constructed as a differential pressure arrangement. The device has two mutually oppositely operating working spaces and is free of forces when both spaces are acted upon by pressure. As a result, the sheet holding force can be reduced by the targeted admission of pressure to one working space and can be increased when this pressure space is relieved from pressure. By the targeted admission of pressure to the working space, which generates a force directed in the moving direction of the pressure box, measures can be taken against the rise of the sheet holding force by way of the drawing course. Thereby, the sheet holding force can be reduced toward the end of the drawing stroke, as is frequently required. Furthermore, during the return stroke of the slide, the drawing device can be blocked or braked. In addition, a damping is possible when the pressure box moves up.
When the pressure box moves up, the working space, which is reduced in this embodiment, can be utilized as a buffer volume if a corresponding control device is configured correspondingly. Toward the end of the return stroke, the corresponding working space will act as a pneumatic buffer, in which case the resulting excess pressure can be utilized for generating compressed air to permit energy recuperation.
The control device of the present invention becomes particularly simple if, for admitting pressure to the working space, whose force acts in the working stroke direction (moving direction of the pressure box during the drawing operation), a pressure level is used which has a defined height. Correspondingly large buffer volumes prevent excessive pressure changes during the movement of the pressure box so that the sheet holding force depends essentially only on the connected pressure level.
In another particularly advantageous embodiment of the present invention, the pressure box is provided with several hydraulic cylinders which act upon the studs for transmitting force to the sheet holding ring. The hydraulic cylinders can be controlled individually or in groups. Advantageously, a control device is used for this purpose which contains no proportional valves but switching valves, which when mounted on the pressure box, can better withstand the occurring shock-related stress. The switching valves permit at least two operating modes, whereby, in one of the two operating modes, the hydraulic cylinders of one group are connected with the hydraulic cylinders of at least one other group so that a pressure compensation is possible. As a result, local force fluctuation, for example, because of different stud lengths, can be compensated or excluded. In the other operating mode, the hydraulic cylinders are individually or in groups switched to a different pressure level or, for example, switched to a no-pressure state. In this manner, the sheet holding force can be reduced locally so that, for influencing the flow of material, a force distribution can be adjusted in a targeted manner along the sheet holding ring. A functional separation is therefore established between the generating and the time-dependent controlling of the sheet holding force overall and the adjusting of the local distribution of the sheet holding force. The overall force is applied by the pneumatic driving device, while the force distribution is adjusted by the hydraulic cylinders.
Advantageously, a carrier or force transmitting device (e.g., a slightly elastic plate) is arranged between the hydraulic cylinders and the studs. Thus, the number of the studs can exceed the number of hydraulic cylinders. With a defined number of studs, a comparatively lower number of hydraulic cylinders will be sufficient which reduces the constructional expenditures and the control expenditures.
When individual hydraulic cylinders are combined to groups, the control expenditures are reduced. The groups are preferably arranged such that, with sheet holding rings of different sizes, a force distribution becomes possible whose stages are as small as possible. This is achieved in that the groups are arranged essentially radially starting from a center point of the pressure box.